It is well established that living organisms have internal biological clocks which regulate activities such as their sleep/wake cycles. These biological clocks are expressions of the effects of one or more endogenous pacemakers thought to be located in the suprachiasmatic nuclei (SCN) of the hypothalamus. The activity sequences defined by these internal biological clocks are referred to as circadian rhythms. Different organisms have different circadian cycles. Creatures which tend to be active during periods of daylight and inactive at night are termed diurnal. Creatures which are active at night and sleep during the day are referred to as nocturnal. In general, natural circadian rhythms are entrained by and tend to follow the natural sequence of daytime light and nighttime darkness which occurs as the earth rotates. For further information see Moore, R. Y., in Sleep and Biological Rhythms, (Montplasir, J. & Godbout, R., eds.), pages 3-10, Oxford Univ. Press (1990).
Under normal circumstances, the circadian rhythm of humans serves as a useful time regulator of various activities. In some instances, however, the internal circadian clock interferes with desired adaptations to differing time schedules. For example, air travelers who rapidly cross two or more time zones may find their internal circadian clocks out of phase with the day/night cycle at their destination, giving rise to the so-called "jet-lag" syndrome in which they suffer disruptions of their sleep patterns and diminished attention span and alertness until their inner biological clocks gradually adjust to local time. Shift workers, whose work schedules rotate among day shift, night shift and the so-called "graveyard" shift, may experience transient internal temporal dissociation or a lack of synchronization among various bodily rhythms, and consequent difficulty in adjusting to shift changes. This can not only adversely affect worker productivity, but in some instances may raise safety concerns. In addition, some persons suffer from circadian irregularities such as insomnia or advanced or delayed sleep phase syndrome which interfere with maintenance of normal activity pattern.
Various approaches have been attempted to adjust circadian rhythms. For example, it has been discovered that exposure to sufficiently bright light at appropriate times in the circadian cycle can advance or delay the circadian rhythm. In addition, attempts have been made to use chemical agents, such as melatonin, to advance or delay the circadian rhythm of an organism, see Lewy et al., Chronobiology International, Vol. 9, No. 5, pp 380-392 (1992). Despite much effort in the prior art, however, there has remained a substantial need for methods to adjust the circadian rhythm of a mammal.